1. Field of the Invention
This invention relates to a process of electrolyzing aqueous solutions of alkali halides in membrane cells in which pH values above 1.0 are maintained in the anode chamber and the alkali solution is moved through the anode chamber and through zones in which it is strengthened to increase its alkali halide concentration and its pH value is adjusted.
2. Discussion of the Prior Art
Mainly two processes, namely, the amalgam and the diaphragm process, have been available until recently for the electrolysis of alkali halide solutions, mainly sodium chloride solutions. The amalgam process affords the advantage of permitting of manufacture of highly concentrated alkaline solutions of high purity for pollution control. The diaphragm process does not involve such expenditures but permits only of a production of alkaline solutions having a much lower concentration and having considerable contents of alkali halides.
When it had become possible to provide so-called ion exchange membranes which are chemically resistant, hydraulically impermeable and permeable substantially only to cations, membrane processes using such ion exchange membranes gained increased importance. It is believed that the membrane process will be the process of the future.
In the membrane process the anode and cathode chambers of the electrolytic cell are separated by an ion exchange membrane, through which substantially only the alkali metal ions can permeate. These ions are electrically neutralized at the cathode and when contacted with water in the anode chamber form alkaline hydroxide solution and hydrogen. Something halogen gas is electrolitically formed released only in the anode chamber.
When the presently known ion exchange membranes are used, part of the OH ions formed in the cathode chamber inevitably migrate through the membrane into the anode chamber. For instance, in the electrolysis of sodium chloride the losses in efficiency amount to 5 to 20% depending upon the quality of the membrane and its time of use. The entrance of OH ions into the anode chamber accounts for about 2/3 of such losses. In the anode chamber, the OH ions in contact with halogen gas form oxyacids of chlorine or the salts of such acids, particularly hypochlorite and chlorate, depending on the pH value of the anolyte, and these acids or salts can be destroyed only by addition of acid.
The formation of oxyacids of chlorine and/or the salts of such acids is also responsible for the decrease of the solubility of the alkali halide. As a result, the discharge potential is shifted toward the less noble side, in an extreme case by about 50 mV.
To eliminate the disadvantage which has been described hereinbefore, it is known to use an acid anolyte in a membrane cell for electrolysis and to add so much hydrochloric acid to the brine that the entering ions are just neutralized (D. Bergner "Elektrolytische Chlorerzeugung nach dem Membran-Verfahren", Chemiker-Zeitung 101 (1977), pages 433 to 447). It is also known to add so much acid that the anolyte is adjusted to a pH value in the range of about 1 to 5, preferably between 3.0 and 4.0, (West German Offenlegungsschrift No. 24 09 193) or about 2 (West German Offenlegungsschrift No. 26 31 523). In the process disclosed in U.S. Pat. No. 3,948,737, the pH value of the anolyte should not exceed 4.5 and should preferably lie between 2.5 and 4.0 although pH values of below 1.0 are also permitted.
Whereas an adjustment to a low pH value is recommendable for destruction of oxyacids of chlorine and of the salts of such acids, very high hydrogen ion concentrations have the disadvantage that they cause hydrogen ions to migrate through the membrane into the cathode chamber where they react with the alkaline solution and thus decrease the current efficiency with respect to the production of such alkaline solution. It is apparent that the current efficiency is thus increased as far as the production of halogen is concerned but decreased with respect to the production of alkaline solution (D. Bergner, l.c.).
It is object of this invention to provide a process which can be carried out in a simple manner and which avoids the disadvantages of the known processes and produces desirable results in respect of the yields of both halogen and alkali metal hydroxide solution are concerned.